ZSM-48 is a zeolite with orthorhombic or pseudo-orthorhombic symmetry having ten-ring non-interconnecting, linear channels whose ideal dimensions are 5.5×5.6 Å. ZSM-48 has shown attractive properties as a catalyst for the dewaxing of hydrocarbon feedstocks, see for example U.S. Pat. Nos. 5,075,269 and 6,884,339 and International Publication WO 01/64339. As a result, there is considerable interest in finding new methods of synthesizing ZSM-48 and, in particular, ZSM-48 with high acid activity (that is, low silica/alumina molar ratio).
ZSM-48 was first synthesized by Rollmann et al. using a C4 to C12 organic diamine as a structure-directing agent, see U.S. Pat. No. 4,423,021. As synthesized by Rollman et al., the zeolite contained little or no aluminum and so had little acid activity.
Synthesis of ZSM-48, again with a high silica to alumina molar ratio, in the presence of a mixture of a C2 to C12 alkylamine and a C3 to C5 tetramethylammonium compound is described in U.S. Pat. No. 4,397,827 and in the presence of bis(N-methylpyridyl)ethylinium cations in U.S. Pat. No. 4,585,747.
U.S. Pat. No. 5,961,951 discloses synthesis of ZSM-48 in the presence of ethylene diamine as the structure-directing agent. However, the broad definition of the reaction mixture requires a silica/alumina molar ratio of at least 100 and the only example for which the composition of the ZSM-48 product is cited, Example 2, reports the silica/alumina molar ratio as being 170.
European Patent Publication EP-A-142317 discloses synthesis of ZSM-48 in the presence of certain linear diquaternary compounds having the general formula:[(R′)3N+(Z)m[(R′)3N+](X−)2 in which each R′ is an alkyl or heteroalkyl group having from 1 to 20 carbon atoms, a cycloalkyl or cycloheteroalkyl group having from 3 to 6 carbon atoms or an aryl or heteroaryl group; Z is an alkylene or heteroalkylene group having from 1 to 20 carbon atoms, an alkenylene or heteroalkenylene group having from 2 to 20 carbon atoms or an arylene or heteroarylene group; m is 5, 6, 8, 9 or 10 and X− is an anion. EP-A-142317 reports that the silica/alumina molar ratio of the reaction mixture must be at least 100 since at lower values a different silicate framework is produced.
U.S. Pat. No. 6,923,949 discloses pure phase ZSM-48 crystals having a XO2/Y2O3 ratio of less than about 150/1, where X is at least one of Si or Ge, preferably Si, and Y is at least one of Al, Ga, B, Fe, Ti, V and Zr, preferably Al, and free from ZSM-50 and Kenyaite impurities having a diameter of less than about 1 micron and being substantially free of fibrous morphology. The material is produced by crystallizing a reaction mixture comprising at least one organic template material selected from organic linear diquaternary alkyl ammonium compounds and linear diamino alkanes, and heterostructural zeolite seeds selected from ZSM-5, ZSM-11, ZSM-12, colloidal BEA, Beta, X and Y zeolites. The Examples employ hexamethonium chloride as the linear diquaternary alkyl ammonium compound and produce ZSM-48 crystals with a SiO2/Al2O3 molar ratio as low as 67.7.
International Publication No. WO 2007/070521 discloses synthesis of a composition comprising ZSM-48 crystals having a silica:alumina molar ratio of 110 or less that is free of non ZSM-48 crystals and free of ZSM-50 by crystallizing a reaction mixture comprising an aqueous mixture of silica or silicate salt, alumina or aluminate salt, hexamethonium salt and alkali base wherein the reaction mixture has the following molar ratios: silica:alumina molar ratio from 70 to 110, base:silica from 0.1 to 0.3 and hexamethonium salt:silica from 0.01 to 0.05. Production of ZSM-48 crystals with a silica:alumina molar ratio as low as 80 are exemplified.
In an article entitled “Reinvestigation into the synthesis of zeolites using diquaternary alkylammonium ions (CH3)3N+(CH2)nN+(CH3)3 with n=3−10 as structure-directing agents”, Microporous and Mesoporous Materials, 68 (2004), 97-104, Song-Ho Lee et al. describe the effects of synthesis variables, especially, the type and concentration of alkali cations on the phase selectivity of zeolite crystallization in the presence of Me6-diquat-n ions where n varies between 3 and 10. In particular, Song-Ho Lee et al. report in Table 2 that with a synthesis mixture containing Me6-diquat-5 and having an silica/alumina molar ratio of 60 and a diquat/silica molar ratio of 0.1, crystallization produces ZSM-48 when the OH−/SiO2 molar ratio of the mixture is 0.33 or less but produces ZSM-12 at an OH−/SiO2 molar ratio of 0.47 and MCM-22 at OH−/SiO2 molar ratios of 0.6 and 0.73. At even higher OH−/SiO2 molar ratios, the product is mordenite and/or analcime.
According to the invention, it has now been found that, in the synthesis of high activity ZSM-48 using Me6-diquat-5 as a structure-directing agent, the diquat/silica molar ratio is a critical variable impacting the phase selectivity of the product. Moreover, it has been found that by using a combination of Me6-diquat-5 and Me6-diquat-6 as the structure-directing agent, it is possible to exercise control over the morphology of the resultant ZSM-48 crystals.